Wireless communication systems, such as the cellular radio telephone systems, typically include subscriber units (such as mobile or portable units) which communicate with a fixed network communication unit via radio frequency (rf) transmissions. A typical fixed communication network includes at least a base station and a switching center. The switching center a subscriber unit accesses may not be its "home" switching center. In this case, the subscriber unit is termed a roamer. The switching center it accesses (termed the "visited" switching center) will communicate with the subscriber unit's "home" switching center via the public switched telephone network (PSTN). One responsibility of the fixed network communication unit is to grant use of the communication system to the subscriber unit after the requesting subscriber unit meets the authentication requirements of the system. In a typical cellular telephone communication system, each subscriber unit is assigned a telephone number (mobile identification number) (MIN) and a unit identification number (or electronic serial number) (ESN) which together serve as a unique identifier for the subscriber to any fixed network communication unit. The fixed network communication unit has access to these identification numbers through a database. Often these numbers are used by the fixed network communication units to bill subscribers for the time the subscriber uses the system. In the case of a roaming subscriber unit, the "visited" switching center must communicate with the subscriber's "home" system database to authenticate and bill the subscriber unit.
Detection of a legitimate subscriber's identification number may be accomplished by rf eavesdropping or by purposeful or inadvertent divulgence of the MIN/ESN combination by the radio telephone installer. Once the subscriber's telephone number and identification number is known (stolen), a thief may reprogram another subscriber unit with the stolen identification number causing two or more subscriber units to have the same MIN/ESN combination. These unauthorized, reprogrammed subscriber units are referred to as "clones." These clones may cause a number of problems for both the system operator and legitimate subscriber unit. A lot of air time, including long distance calls, could be charged against the legitimate subscriber; if undetected the legitimate subscriber will pay for these calls, and if detected (typically) the operator absorbs the costs. Additionally, it is possible that the clone's registrations could disrupt normal call delivery to the legitimate subscriber, and heavy usage could block calls by the legitimate subscriber.
In response to the problem of cloning several authentication schemes have been proposed. However, all of these schemes require additional programming or equipment and cannot be readily implemented with the millions of existing analog subscribers whose only means of authentication is the MIN/ESN. Another solution has been to maintain a database on subscribers with their usage patterns and analyze new calls against the stored pattern for possible fraud. However, this approach is passive only, waiting until a call is initiated, and is limited by the established pattern. The broader the pattern established by the legitimate subscriber, the easier it is for a clone to go undetected; the narrower the pattern, the more likely the legitimate subscriber will be inconvenienced by his own use outside the pattern. So, while effective anti-cloning schemes are being implemented for newer subscriber units, there remains a need for an active anti-cloning system that can be readily implemented with all subscriber units.